There are two things of concern here: precedence and fixity. As sepp2k mentioned, this question on Stack Overflow explains the precedence, thought the rules, as quoted, are not complete enough, and there were very small changes from Scala 2.7 to Scala 2.8. Differences concern mostly operators ending in =, though.

As for fixity, almost everything in Scala is read left to right, which is what programmers are used to. In Scala, however, operators ending in : are read right to left.

Take, then, this example:

1 + 2 :: Nil

First, precedence. What has most precedence, + or :? According to the table, + has precedence over :, so the addition is done first. Therefore, the expression is equal to this:

((1).+(2)) :: Nil

Now there's no precedence conflict, but since :: ends in :, it has a diferent fixity. It is read right to left, therefore:

Nil.::((1).+(2))

On the other hand, in this:

gen nextInt 3 :: Nil

The operator :: has precedence over nextInt, because : has precedence over all letters. Therefore, and remembering its fixity, it becomes:

gen nextInt Nil.::(3)

Which then becomes

gen.nextInt(Nil.::(3))

At which point the error is obvious.

PS: I'm writing (1).+(2) instead of 1.+(2) because, at the time of this writing, 1. is interpreted as a double number, making 1.+(2) an infix expression adding the double 1.0 to 2. This syntax is deprecated as of Scala 2.10.0, and will probably not be present on Scala 2.11.